Hierarchical photonic crystals and methods of making the same

ABSTRACT

The present disclosure provides a composition comprising a hierarchical opal that exhibits structural color when exposed to incident electromagnetic radiation. The hierarchical opal comprises nanoscale periodic cavities separated by a lattice constant, and includes a surface having grooves. The grooves may form a diffractive optical element on the surface of the hierarchical opal, such as a diffuser, a diffraction grating, a beamsplitter, a beam displacement opic, a Fresnel lens, and a micro lens, among others.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority to U.S. Provisional Application No.62/768,646 filed Nov. 16, 2018, and U.S. Provisional Application No.62/837,693 filed Apr. 23, 2019, the contents of each of which areincorporated by reference.

GOVERNMENT SUPPORT

This invention was made with government support under grantN00014-16-1-2437 awarded by the United States Navy. The government hascertain rights in the invention.

BACKGROUND

Photonic crystals include periodically repeating internal regions ofhigh and low dielectric constants. Photons propagate through thestructure based upon the wavelength of the photons. Photons withwavelengths of light that are allowed to propagate through the structureare called “modes,” while photons with wavelengths of light that are notallowed to propagate are called “photonic band gaps.” The structure ofthe photonic crystals define allowed and forbidden electronic energybands, resulting in spectral selectivity of light.

SUMMARY

In some embodiments, the present disclosure provides a method of forminga hierarchical opal configured to separate wavelengths of light using adiffractive optical element. The method includes applying a silk fibroinsolution to a lattice comprising a plurality of particles to fill voidsbetween the plurality of particles. The lattice forming a mold for thehierarchical opal having the diffractive optical element. The methodfurther includes drying the silk fibroin solution into a compositematerial including the hierarchical opal and the plurality of particles,and removing the plurality of particles to form the hierarchical opalhaving the diffractive optical element formed on a surface of thehierarchical opal.

In some embodiments, the present disclosure provides a method of forminga hierarchical opal. The method includes applying the silk fibroinsolution to a lattice comprising a plurality of particles such that thesilk fibroin solution fills voids between the plurality of particles,and drying the silk fibroin solution into a composite material includingthe hierarchical opal and the plurality of particles. The method furtherincludes removing the plurality of particles to form the hierarchicalopal comprising nanoscale periodic cavities separated by a latticeconstant. The method further includes applying an aqueous solution to asurface of the hierarchical opal in a patterned formation, where theaqueous solution alters the lattice constant of the nanoscale periodiccavities located in the patterned formation to generate grooves acrossthe surface of the hierarchical opal.

In some embodiments, the present disclosure provides an apparatuscomprising a hierarchical silk fibroin opal that exhibits structuralcolor when exposed to incident electromagnetic radiation. Thehierarchical silk fibroin opal comprises nanoscale periodic cavitiesseparated by a lattice constant. The hierarchical silk fibroin opalincludes a surface having grooves. The grooves may form a diffractiveoptical element on the surface of the hierarchical silk fibroin opal.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures depict certain illustrative implementations of thecompositions, apparatuses, and methods described herein, where likereference numerals refer to like elements. Each depicted implementationis illustrative of the compositions, apparatuses, and methods and shouldnot be construed to be limited.

FIG. 1 is a flow chart illustrating an example method of forming abiologically-based hierarchical opal in accordance with some embodimentsof the present disclosure.

FIG. 2 is a schematic illustrating a non-limiting example method offorming the biologically-based hierarchical opal in accordance with someembodiments of the present disclosure. Left to right: introducingmonodisperse polystyrene (PS) nanospheres to water surface;crystallization at the water/air interface; forming hierarchical PScolloidal crystals with controllable layers through layer-by-layertransferring the crystalline monolayer to patterned substrate; casting abiocompatible polymer solution into the template and drying to generatepatterned biocompatible polymer/PS composite; removing PS nanospheres toattain hierarchical inverse colloidal crystal.

FIG. 3 is a flow chart illustrating an example method of forming abiologically-based hierarchical opal in accordance with some embodimentsof the present disclosure.

FIG. 4 is a schematic illustrating two printing patterning processes inaccordance with some embodiments of the present disclosure. Top panelleft: monodisperse PS spheres are introduced; Top panel middle: form acrystalline monolayer at the water/air interface; Top panel right:repeat steps to get a 3-layer stacked crystal structure; Middle panelright: ethyl acetate is deposited on PS structure by inkjet printing tolocally dissolve PS beads as pattern designed; Middle panel middle:aqueous biocompatible polymer solution casting to form biocompatiblepolymer/PS composite; Middle panel left: patterned biologically-basedhierarchical inverse opal is obtained by immersing the composite intotoluene to dissolve and remove PS. Bottom panel: water/MeOH mix solutionis deposited on the biologically-based hierarchical inverse opal tocreate grooves or a defected porous structure.

FIG. 5(A-D) are surface morphology images of the biologically-basedhierarchical opals in accordance with some embodiments of the presentdisclosure. Panel A (left) is a low-magnification surface SEM image of adiffuser patterned hierarchical opal; Panel A (right) is ahigh-magnification surface SEM image of a diffuser patternedhierarchical opal; Panel B (left) is a low-magnification surface SEMimage of a hierarchical opal having a pattern generator structure; PanelB (right) is a high-magnification surface SEM image of a hierarchicalopal having a pattern generator structure; Panel C (left) is alow-magnification surface SEM image of a hierarchical opal having alinear grating with a period of 1.6 μm; Panel C (right) is ahigh-magnification surface SEM image of a hierarchical opal having alinear grating with a period of 1.6 μm; Panel D is a 3D atomic forcemicroscopy (AFM) image of the grating template.

FIG. 6(A-C) are surface SEM images of hierarchical opals in accordancewith some embodiments of the present disclosure. Panel A are SEM imagesof a diffuser (D-HOP); Panel B are SEM images of a pattern generator(PG-HOP); and Panel C are SEM images of a grating (DG-HOP).

FIG. 7(A-B) are 2D profiles of the surface of a template and thehierarchical opal in accordance with some embodiments of the presentdisclosure. Panel A is a cross-section profiles of 2D-diffractivesurfaces of a template, and panel B is a replicated inverse opalprofile.

FIG. 8(A-C) are cross-sectional SEM images of HOPs formed using by usingtemplates of colloidal crystals composed of PS spheres with diameter of300 nm on different topographically patterned substrates in accordancewith some embodiments of the present disclosure. Panel A is a diffuser(D-HOP); Panel B is a pattern generator (PG-HOP); Panel C is a grating(DG-HOP).

FIG. 9(A-I) illustrate integration of structural color and diffractionin the hierarchical opal in accordance with some embodiments of thepresent disclosure. Panels A-B illustrate total reflectance spectra ofD-HOP (Panel A), and PG-HOP (Panel B) with Λ=300 nm. The spectra ofcorresponding 2D optical elements are also shown for comparison. Scalebars: 2 mm. Panel C-D illustrate corresponding 3D AFM images of D-HOP(Panel C) and PG-HOP (Panel D). Panels E-F illustrate the projecteddiffraction patterns in both reflection and transmission modes obtainedfrom propagation of a green laser (λ=543.5 nm) through D-HOP (Panel E)and PG-HOP (Panel F). Panel G illustrates a schematic of theexperimental setup to generate transmitted images of the “Tufts” wordsthrough HOPs. Double arrow indicates the separation between the actualimage and the sample. H-I) Corresponding transmitted images of the“Tufts” words through D-HOP (Panel H) and G-HOP (Panel I) with Λ=300 nm.The arrows in Panel H indicate the visible letters through silk inverseopal film. In I, the “Tufts” word in the middle is diffracted to bothsides by G-HOP. Scale bars: 2 mm.

FIG. 10(A-I) illustrate diffraction performance modulation ofhierarchical opals in accordance with some embodiments of the presentdisclosure. Panel A) reflected (top) and transmitted (bottom)diffraction patterns of PG-HOPs with different layers of inversecolloidal crystals (Λ=300 nm) illuminated by a green laser. Panel B)Calculated diffraction, reflection and transmission intensity of PG-HOPsas a function of layer numbers of inverse colloidal crystal. Panel C)Measured+1^(st) order diffraction efficiency of G-HOPs as a function oflayer numbers. The samples were illuminated by a green laser. Insert:transmitted diffraction pattern showing different diffraction orders.Panel D) Reflected diffraction patterns of five-layer D-HOPs with Λ=210nm (1,2) or Λ=300 nm (3,4) illuminated by a blue (1,3) or green (2,4)laser. Panels E-F are calculated reflection and diffraction intensity asa function of UV (Panel E) and water vapor (Panel F) treatment time.Inserts show the corresponding reflected diffraction patterns generatedby using a green laser illumination. Panel G is a photograph showing thesplit of a white light beam into its component colors of a G-HOP filmwith Λ=300 nm. Scale bar: 2 mm. Panel H is a transmitted diffractionpatterns obtained by shining a white light beam through grating andfive-layer G-HOPs. Panel I is the transmission intensity (+1^(st) order)difference between grating and G-HOPs (I_(G)−I_(G-HOP)) as a function ofwavelength.

FIG. 11(A-F) illustrates structural color tuning at different angles inaccordance with the present disclosure. Panel A) Side-view photograph ofD-HOP with Λ=300 nm. The arrow indicates the inverse opal structuresurrounding the D-HOP. Panel B) Reflectance spectra of D-HOP measuredunder diffusive reflection mode with different detection angle (θ),insert shows the diagram of diffusive reflection measurement system.Panel C) Schematic (left) and photographs (right) of reflectedstructural coloration on the grating and G-HOPs with Λ=300 nm atdifferent angles. Panels D-F) Broad angle pattern display. Panels D-E)Shadow masks designs (left) and the corresponding photograph ofpatterned HOPs (right). A “silk” word (Panel D) and a tree (Panel E)pattern is created by selectively exposing part of D-HOP to UV light andPG-HOP to water vapor, respectively. Panel F) Schematic diagram andphotographs of the PG-HOP with a tree pattern observed at the anglesfrom 15° to 45°. θ is defined as viewing angle. The photographs of asilk inverse opal with a tree pattern observed at different angles arealso shown for comparison. Scale bars: 2 mm.

FIG. 12(A-D) illustrate applications of the HOPs for sensing. PanelsA-B) Colorimetric sensing in accordance with some embodiments of thepresent disclosure. Panel A) Stop-band response of a nine-layer D-HOPwith Λ=210 nm to IPA-glycerol mixtures with varying compositions.Inserts show the structural color change when the glycerol concentrationvaries from Φ=0 to Φ32 0.8. Panel B) Calculated stop-band centralwavelength and intensity as a function of the volume fraction ofglycerol, Φ. Panel C, Panel D) Diffraction-based sensing. Panel C)Transmitted diffraction patterns of diffuser and D-HOPs immersed indifferent IPA-glycerol mixtures. Panel D) Relations between normalizeddiffraction intensity and the volume fraction of glycerol.

FIG. 13(A-D) illustrate example images of patterned hierarchical opalsusing ethyl acetate as the ink in accordance with embodiments of thepresent disclosure. Panel A) Direct and inverse patterns can both becreated simply by changing printing files; Panel B) A 5 cm×5 cm coilpattern, in which the distance between printed lines is around 110 μm;Panel C) Series of thin lines; Panel D) Pixelated Lena patterned on SIOand SEM images of the Lena pattern. Boundary between destroyed andintact area can be distinguished.

FIG. 14(A-D) illustrate example images and graphs of QR code patternedon hierarchical opals by printing ethyl acetate in accordance with someembodiments of the present disclosure. Panel A) Patterned QR code on apiece of water-soluble free-standing SIO, which indicates the device'sability in message disclosure prevention; Panel B) Reversed QR pattern;Panel C) Normalized reflectance spectra of the original SIO and the onesthat are WV treated. The central peak position changed from 600 nm(control) to 550 nm, 485 nm, and 445 nm after WV treatment for 1, 2, and3 seconds respectively; and Panel D) Decipher process of WV treated B),the QR code is reversed and taken as the 1^(st) encryption layer, whilethe parts treated with WV are translated into key codes 796 according tothe custom criteria set to RGB intensity. The scale bar is the same for(A), (B) and (D).

FIG. 15(A-E) illustrate example images, SEM images, and graphs ofpatterned hierarchical opals using MeOH/water mixture as ink inaccordance with some embodiments of the present disclosure. Panel A)(a-f) are jumbos (1 cm in width) with different colors on 10-layer-SIOfilms by changing MeOH/water composition (firing volt: 25V, water %: 3,3.5, 5, 9.5, 12, 18 respectively); Panel B) Cross section SEM images of10-layer control, A(c) and A(f) (areas indicated in white square) toshow structure shrinkage as a response to different amount of water;Panel C) Real reflectance spectra of 10-layer control and (a-f)respectively; Panel D) Linear relationship is shown between the watercontent in MeOH/water mix and peak wavelength of the reflective spectrademonstrated in C); Panel E) Butterfly pattern with two colors (body,veins and wing periphery) generated on a 10-layer-SIO film.

DETAILED DESCRIPTION

Among other things, the present disclosure provides biologically-basedhierarchical opals, and specifically silk fibroin-based hierarchicalopals, as well as methods of preparing the biologically-basedhierarchical opals. Various embodiments according to the presentdisclosure are provided in detail herein.

Methods of Manufacturing Hierarchical Opals

In some embodiments, the present disclosure provides a method of formingbiologically-based hierarchical opals, and specifically silkfibroin-based hierarchical opals. Referring to FIGS. 1-2, an examplemethod 100 for forming a biologically-based hierarchical opal 26 isillustrated in accordance with some embodiments of the presentdisclosure. The method 100 includes preparing a biocompatible polymersolution 102. The biocompatible polymer solution may include abiocompatible polymer at a concentration from 0.1% (w/w) to 50% (w/w),or from 1% (w/w) to 15% (w/w), or from 4% (w/w) to 10% (w/w) of thesolution.

As indicated by step 104, the method 100 includes inducing a pluralityof particles 10 to assemble into a lattice 12 (e.g., crystallizedparticles). The lattice 12 may be formed by applying a layer ofparticles 10 to a surface 16 of a substrate 14. In some embodiments, theparticles 10 may be deposited onto the substrate 16 at a liquid/airinterface 18. The liquid may be allowed to evaporate at ambientconditions to induce self-assembly and crystallization of the particles10, or the liquid may be heated until the liquid is removed to yield thelattice 14. In some embodiments, the process of casting particles 10onto the surface 16 of the substrate 14 is repeated by layer-by-layerdeposition to form a hierarchical lattice 20. As used herein, the term“hierarchical” refers to a three-dimensional structure having apatterned surface (e.g., the surface of the hierarchical lattice 18 maycomprise a plurality of grooves, ridges, and/or channels 22). In someembodiments, the hierarchical lattice 20 includes multiple layers ofparticles 10 (e.g., one layer, two layers, three layers, four layers,five layers, ten layers, twenty layers, fifty layers, one hundredlayers, five hundred, or more).

As shown in FIG. 2, the surface 16 of the substrate 14 may be patternedwith the hierarchical structure, for example, the surface 16 of thesubstrate 14 may include a plurality of grooves, ridges, and/or channels22. By performing layer-by-layer deposition of particles 10 on thepatterned substrate 14, the surface of the hierarchical lattice 20 mayconform or substantially match the surface of the patterned substrate14. In some embodiments, the hierarchical lattice 20 forms a negativeimprint of the patterned surface of the substrate 14. Referring back toFIG. 1, the method 100 further includes a step 106 of applying thebiocompatible polymer solution to the hierarchical lattice 20 such thatthe biocompatible polymer solution fills voids between the plurality ofparticles 10. The biocompatible polymer solution is then dried for aduration (e.g., 24 h at 25° C. and 30% relative humidity) to form a film24 (e.g., biocompatible polymer/particle composite) that encloses theparticles 10.

As indicated by step 110, the particles 10 are then removed, leavingperiodic cavities or voids 28 in the film 24 to form a hierarchical opal26 having a structural color. In some embodiments, the particles 10comprise polystyrene particles or poly(methyl methacrylate) particles.The particles 10 may be removed by exposing the film 24 to an organicsolvent that selective dissolves the particles 10, but does not dissolvethe film 24. Suitable solvents include, but are not limited to, tolueneor ethyl acetate. As used herein, the term “structural color” refers tocolors caused by interference effects rather than by pigments.Structural color is caused by the interaction of light with structuresof nanoscale periodic structure, with geometries on the order ofmagnitude of visible light wavelengths. Light that encounters theseminute structures is subject to optical phenomena including thin filminterference, multilayer interference, diffraction grating effects,photonic crystal effects, and light scattering. These phenomena lead toselective reflection of particular light wavelengths throughconstructive and destructive interference. In some embodiments, thebiologically-based hierarchical opal 26 can be structurally manipulatedto diffract light of a particular wavelength, resulting in perceivedcolor. For example, the structural color may be adjusted based on anumber of factors, including the diameter of the periodic cavities orvoids 28 and a lattice constant. As used herein, the term “latticeconstant” refers to a center-to-center distance of the periodic cavities28 in the hierarchical opal 26.

In some embodiments, the particles 10 and periodic cavities 28 may havean average diameter that ranges from about 5 nm to about 2000 nm, ormore. For example, the particles 10 and periodic cavities 28 may have anaverage diameter of at least about 5 nm, at least about 10 nm, at leastabout 15 nm, at least about 20 nm, at least about 25 nm, at least about30 nm, at least about 35 nm, at least about 40 nm, at least about 50 nm,at least about 60 nm, at least about 70 nm, at least about 80 nm, atleast about 90 nm, at least about 100 nm, at least about 125 nm, atleast about 150 nm, at least about 175, at least about 200 nm, at leastabout 225 nm, at least about 250 nm, at least about 275, at least about300 nm, at least about 325 nm, at least about 350 nm, at least about375, at least about 400 nm, at least about 425 nm, at least about 450nm, at least about 475, at least about 500 nm, at least about 525 nm, atleast about 550 nm, at least about 575, at least about 600, at leastabout 650, at least about 700, at least about 750, at least about 800,at least about 850, at least about 900, at least about 950, at leastabout 1000, or more.

In some embodiments, the periodic cavities 28 may have an averagediameter that is at most 2000 nm, or at most 1500 nm, or at most 1400nm, or at most 1300 nm, or at most 1200 nm, or at most 1100 nm, or atmost 1000 nm, or at most 900 nm, or at most 800 nm, or at most 700 nm,or at most 600 nm, or at most 500 nm, or less.

In some embodiments, the lattice constant of the hierarchical opal 26may range from about 5 nm to about 2000 nm, or more. For example,lattice constant may be at least about 5 nm, or at least about 10 nm, orat least about 15 nm, at least about 20 nm, at least about 25 nm, atleast about 30 nm, at least about 35 nm, at least about 40 nm, at leastabout 50 nm, at least about 60 nm, at least about 70 nm, at least about80 nm, at least about 90 nm, at least about 100 nm, at least about 125nm, at least about 150 nm, at least about 175, at least about 200 nm, atleast about 225 nm, at least about 250 nm, at least about 275, at leastabout 300 nm, at least about 325 nm, at least about 350 nm, at leastabout 375, at least about 400 nm, at least about 425 nm, at least about450 nm, at least about 475, at least about 500 nm, at least about 525nm, at least about 550 nm, at least about 575, at least about 600, atleast about 650, at least about 700, at least about 750, at least about800, at least about 850, at least about 900, at least about 950, atleast about 1000, at least about 1500, or at least about 2000 nm ormore.

In some embodiments, the lattice constant of the hierarchical opal 26may have an average diameter that is at most 2000 nm, or at most 1500nm, or at most 1400 nm, or at most 1300 nm, or at most 1200 nm, or atmost 1100 nm, or at most 1000 nm, or at most 900 nm, or at most 800 nm,or at most 700 nm, or at most 600 nm, or at most 500 nm, or less.

In some embodiments, the surface structure of the hierarchical opal 26may be tunable and configured to induce a variety of optical effectsincluding, but not limited to, iridescense, angle-independentcoloration, polarization, complex color mixing, antireflection,ultra-blackness, ultra-whiteness, light focusing, and dynamic structuralcolor. In additional to coloration, the grooves 22 on the surfacestructure of the hierarchical opal 26 may be configured to add furtherfunctional utility such as superwettability, selective vapor responses,light trapping, diffraction.

In some embodiments, the grooves, ridges, and/or channels 22 on thesurface of the hierarchical opal 26 may be configured to define adiffractive optical element. As used herein, the term “diffractiveoptical element” may refer to a hierarchical opal 26 having a surfacecomprising grooves, ridges, and/or channels 22 arranged to selectivelyshape and/or separate wavelengths of incident light which operate by wayof diffracting electromagnetic radiation. In some embodiments, thehierarchical opal 26 may include a surface that forms a diffractiveoptical element including, but not limited to, a diffuser, a patterngenerator (i.e., an element that diffracts incoming light in a fashiondesigned to generate a specific pre-determined pattern, as would beunderstood by those having ordinary skill in the diffraction patterntransform art), a diffraction grating, a Fresnel lens, a micro lens, abeamsplitter, a hologram, and a phase plate. In some embodiments, only afraction of the surface of the hierarchical opal 26 includes thediffractive optical element. In some embodiments, the entire surface ofthe hierarchical opal 26 includes the diffractive optical element. Insome embodiments, the surface of the hierarchical opal 26 includesmultiple diffractive optical elements (e.g., one optical element, twooptical elements, five optical element, or more), which may be the sameor different.

In some embodiments, the structure of the diffractive optical element ispresent in each layer of the lattice. For example, when the particles 10are removed, each of the layers of periodic cavities that form in theabsence of the particles 10 may each be in the form of the diffractiveoptical element.

In some embodiments, the method 100 of forming the hierarchical opal 26includes forming a first hierarchical silk fibroin opal 26 havinggrooves, ridges, and/or channels 22 configured in at least one layer.The method 100 further includes coupling or adhering the firsthierarchical silk fibroin opal to a second inverse opal though anadhesive layer. In some embodiments, the second inverse opal may includea surface that is flat or substantially flat. In some embodiments, thesecond inverse opal is a silk fibroin inverse opal. In some embodiments,the adhesive layer is an optically clear adhesive. In some embodiments,the adhesive layer comprises a layer of silk fibroin (e.g., silk fibroinfilm) that couples or adheres the first hierarchical silk fibroin opal26 having grooves to the second inverse opal.

In some embodiments, the gratings be separated by a groove spacing. Insome embodiments, the groove spacing in the hierarchical structureranges from about 600 lines/mm to about 3600 lines/mm. In some cases,the groove spacing can be at least about 600 lines/mm, or at least about700 lines/mm, or at least about 800 lines/mm, or at least about 900lines/mm, or at least about 1000 lines/mm, or at least about 1100lines/mm, or at least about 1200 lines/mm, or at least about 1300lines/mm, or at least about 1400 lines/mm, or at least about 1500lines/mm, or at least about 1600 lines/mm, or at least about 1700lines/mm, or at least about 1800 lines/mm, or at least about 1900lines/mm, or at least about 2000 lines/mm, or at least about 2100lines/mm, or at least about 2200 lines/mm, or at least about 2300lines/mm, or at least about 2400 lines/mm, or at least about 2500lines/mm, or at least about 2600 lines/mm, or at least about 2700lines/mm, or at least about 2800 lines/mm, or at least about 2900lines/mm, or at least about 3000 lines/mm, or at least about 3100lines/mm, or at least about 3200 lines/mm, or at least about 3300lines/mm, or at least about 3400 lines/mm, or at least about 3500lines/mm, or at least about 3600 lines/mm.

In some embodiments, the groove spacing in the hierarchical structure 26is at most 3600 lines/mm, or at most about 3200 lines/mm, or at mostabout 2800 lines/mm, or at most about 2400 lines/mm, or at most about2000 lines/mm, or at most about 1600 lines/mm, or at most about 1200lines/mm, or less.

In some embodiments, the grooves, ridges, and/or channels 22 of thehierarchical opal 26 have a groove spacing or width that ranges fromabout 50 nm to about 30 μm. In some embodiments, the groove spacing orwidth may be at least about 50 nm, at least about 60 nm, at least about70 nm, at least about 80 nm, at least about 90 nm, at least about 100nm, at least about 125 nm, at least about 150 nm, at least about 175, atleast about 200 nm, at least about 225 nm, at least about 250 nm, atleast about 275, at least about 300 nm, at least about 325 nm, at leastabout 350 nm, at least about 375, at least about 400 nm, at least about425 nm, at least about 450 nm, at least about 475, at least about 500nm, at least about 525 nm, at least about 550 nm, at least about 575 nm,at least about 600 nm, at least about 650 nm, at least about 700 nm, atleast about 750 nm, at least about 800 nm, at least about 850 nm, atleast about 900 nm, at least about 950 nm, at least about 1000 nm, ormore.

In some embodiments, the grooves, ridges, and/or channels 22 of thehierarchical opal 26 have a groove spacing or width that is at most 30μm, or at most 25 μm, or at most 20 μm, or at most 15 μm, or at most 10μm, or at most 5 μm, or at most 1000 nm, or at most 800 nm, or at most600 nm, or at most 400 nm, or less.

In some embodiments, the groove spacing may be fixed. Alternatively, thegroove spacing may be irregular (i.e., varies throughout along adimension of the hierarchical opal 26).

In some embodiments, the grooves, ridges, and/or channels 22 of thehierarchical opal 26 have a groove depth or height that ranges fromabout 50 nm to about 1 μm. The groove depth or height may be defined asa distance or length from the top of the groove, ridge, or channelrelative to a surface of the hierarchical opal 26. In some embodiments,the groove depth may be at least about 50 nm, at least about 60 nm, atleast about 70 nm, at least about 80 nm, at least about 90 nm, at leastabout 100 nm, at least about 125 nm, at least about 150 nm, at leastabout 175, at least about 200 nm, at least about 225 nm, at least about250 nm, at least about 275, at least about 300 nm, at least about 325nm, at least about 350 nm, at least about 375, at least about 400 nm, atleast about 425 nm, at least about 450 nm, at least about 475, at leastabout 500 nm, at least about 525 nm, at least about 550 nm, at leastabout 575 nm, at least about 600 nm, at least about 650 nm, at leastabout 700 nm, at least about 750 nm, at least about 800 nm, at leastabout 850 nm, at least about 900 nm, at least about 950 nm, or at leastabout 1000 nm.

In some embodiments, the grooves, ridges, and/or channels 22 of thehierarchical opal 26 have a groove depth or height that is at most 3 μm,or at most 2 μm, or at most 1 μm, or at most 800 nm, or at most 600 nm,or at most 400 nm, or less.

As used herein, the term “biocompatible polymer” refers to any polymericmaterial that does not deteriorate appreciably and does not induce asignificant immune response or deleterious tissue reaction, e.g., toxicreaction or significant irritation, over time when implanted into orplaced adjacent to the biological tissue of a subject, or induce bloodclotting or coagulation when it comes in contact with blood.

Exemplary biocompatible polymers include, but are not limited to, apoly-lactic acid (PLA), poly-glycolic acid (PGA),poly-lactide-co-glycolide (PLGA), polyesters, poly(ortho ester),poly(phosphazine), poly(phosphate ester), polycaprolactone, gelatin,collagen, fibronectin, keratin, polyaspartic acid, alginate, chitosan,chitin, hyaluronic acid, pectin, polylactic acid, polyglycolic acid,polyhydroxyalkanoates, dextrans, and polyanhydrides, polyethylene oxide(PEO), poly(ethylene glycol) (PEG), triblock copolymers, polylysine,alginate, polyaspartic acid, silk fibroin, any derivatives thereof andany combinations thereof.

In some embodiments, the step 102 of preparing the biocompatible polymersolution 102 includes preparing an aqueous solution having thebiocompatible polymer at a concentration from about 0.1% (w/w) to about50% (w/w). In some embodiments, the biocompatible polymer is present ata concentration of from at least about 0.1% (w/w) to at least about 5%(w/w), or at least about 6% (w/w), or at least about 7% (w/w), or atleast about 8% (w/w), or at least about 9% (w/w), or at least about 10%(w/w), or at least about 15% (w/w), or at least about 20% (w/w), or atleast about 25% (w/w), or at least about 30% (w/w), or more.

In one non-limiting example, the biocompatible polymer is or comprisessilk fibroin. Silk solutions may be prepared by boiling silk cocoons inan aqueous solution of Na₂CO₃ (e.g., at 0.2 M), and rinsing thoroughlywith water to remove glue-like sericin proteins. Extracted silk is thendissolved in a solvent, for example, LiBr (e.g., at 9.3 M) solution. Thesolution is then dialyzed against distilled water to remove LiBr, whichresults in a final aqueous silk fibroin solution having silk fibroinpresent in the concentration that ranges between 0.1% (w/w) and 30%(w/w).

In some embodiments, the substrate 14 comprises an optically clearmaterial. In some embodiments, the substrate is a silicone wafer. Thesubstrate may be patterned with a grating structure including, but notlimited to an echellete grating, a littrow grating, or a holographicgrating.

In some embodiments, the step of preparing the biocompatible polymersolution 102 includes an additive at a concentration from 0.1% (w/w) to30% (w/w). In some embodiments, the method 100 includes dispersingplasmonic particles in the solution. Suitable plasmonic particles ornanoparticles include, but are not limited to, gold, silver, ruthenium,rhodium, palladium, osmium, iridium, platinum, titanium, aluminum,nickel, fluorine, cerium, tin, bismuth, antimony, molybdenum, chromium,cobalt, zinc, tungsten, polonium, rhenium and copper.

In some embodiments, the method 100 further includes a step of locallytuning a photonic band gap in the hierarchical opal 26. Locally tuningthe photonic band gap may include exposing a portion of the hierarchicalopal 26 to water vapor exposure or ultra violet radiation exposure.

In some embodiments, water and/or moisture affects structural propertiesof silk. In some embodiments, interaction between silk proteins andwater molecules leads either to beta-sheet formation when a film isexposed to water vapor or can cause material dissolution under certainconditions (i.e. an amorphous, alpha-helix dominated silk structure) ifimmersed in water.

In some embodiments, an ability to controllably affect silk structure isused, such as here, to tune a nanoscale lattice of the hierarchical opal26. In some embodiments, when the hierarchical opal 26 are exposed towater vapor, structural color is gradually shifted with an increase ofwater vapor treating time though an alteration of the lattice constantin the target region. A color shift is shown to occur in a few seconds.In some embodiments, exposing provided silk inverse opals to water vaporincludes exposing for about less than one second to about 5 seconds. Insome embodiments, exposure times are less than 1 second, less than 2seconds, less than 3 seconds, less than 4 seconds, less than 5 seconds,less than 6 seconds, less than 7 seconds, less than 8 seconds, less than9 seconds, or about 10 seconds, or less. In some embodiments, watervapor exposure times are less than a time to cause material dissolution.

In some embodiments, exposing the provided silk inverse opals to watervapor includes exposing for at most ten minutes, or at most 9 minutes,or at most 8 minutes, or at most 7 minutes, or at most 6 minutes, or atmost 5 minutes, or at most 4 minutes, or at most 3 minutes, or at most 2minutes, or at most for one minute, or at most for 30 seconds, or atmost 10 seconds, or less.

In some embodiments, silk structure in the hierarchical opal 26 is alsoaffected by exposure to ultraviolet radiation. In some embodiments,defining structural color in the hierarchical opal 26 makes use of silkstructure modification induced by exposure to ultraviolet radiation(UV).

In some embodiments, exposing provided the hierarchical opal 26 to ultraviolet radiation, exposure times may range from about 15 minutes toabout 5 hours. In some embodiments, exposure times are less than 15minutes, less than 30 minutes, less than 45 minutes, less than 1 hour,less than 1.5 hours, less than 2 hours, less than 2.5 hours, less than 3hours, less than 3.5 hours, less than 4 hours, less than 4.5 hours, lessthan 5 hours, less than 5.5 hours, less than 6 hours, less than 7 hours,less than 8 hours, less than 9 hours, less than 10 hours, or more. Insome embodiments, ultra violet exposure times are less than a time tocause photodegradation of silk fibroin.

In some embodiments, exposing the provided hierarchical opal 26 to ultraviolet radiation includes exposing times of at most 24 hours, or at most12 hours, or at most 10 hours, or at most 8 hours, or at most 6 hours,or at most 4 hours, or at most 2 hours, orat most 1 hour, or less.

In some embodiments, a mask or stencil is applied to the hierarchicalopal 26 prior to application of the UV radiation or the water vapor toselectively expose target regions on the hierarchical opal 26. In thismanner, patterns may be created on the hierarchical opal 26, wheretarget regions have varied lattice constants relative to non-exposedregions.

In some embodiments, the hierarchical opal 26 is or comprises a silkfibroin hierarchical opal 26. At least a portion of the hierarchicalopal 26 may be characterized by a percent beta sheet structure withinthe range of about 0% to about 45%. In some embodiments, silkfibroin-based hierarchical opals 26 are characterized by crystallinestructure, for example, comprising beta sheet structure of about 1%(w/w), at least about 2%, at least about 3%, at least about 4%, at leastabout 5%, at least about 6%, at least about 7%, at least about 8%, atleast about 9%, at least about 10%, at least about 11%, at least about12%, at least about 13%, at least about 14%, at least about 15%, atleast about 16%, at least about 17%, at least about 18%, at least about19%, at least about 20%, at least about 21%, at least about 22%, atleast about 23%, at least about 24%, at least about 25%, at least about26%, at least about 27%, at least about 28%, at least about 29%, atleast about 30%, at least about 31%, at least about 32%, at least about33%, at least about 34%, at least about 35%, at least about 36%, atleast about 37%, at least about 38%, at least about 39%, at least about40%, at least about 41%, at least about 42%, at least about 43%, atleast about 44%, or at least about 45%. The beta sheet structure may beinduced by exposing at least a portion of the hierarchical opal 26 towater vapor, UV radiation, or an organic solvent (e.g., methanolimmersion).

In some embodiments, the hierarchical opal 26 includes a portion of thesilk having a beta structure of at most 65% (w/w), or at most 60%, or atmost 55%, or at most 50%, or at most 45%, or at most 40%, or at most35%, or at most 30% (w/w).

Printing Hierarchical Opals

Referring to FIGS. 3-4, an example method 200 for forming abiologically-based hierarchical opal 26 is illustrated in accordancewith some embodiments of the present disclosure. The method 200 includespreparing a biocompatible polymer solution 202. The biocompatiblebiocompatible polymer solution may include a biocompatible polymer at aconcentration of 0.1% (w/w) to 30% (w/w), or from 1% (w/w) to 15% (w/w),or from 4% (w/w) to 10% (w/w) of the solution, and may be prepared asdescribed with respect to method 100. As indicated by step 204, themethod 200 includes inducing a plurality of particles 10 toself-assemble into a lattice 12 (e.g., crystallized particles) or ahierarchical lattice 20. The lattice 12 and the hierarchical lattice 20may be produced as described with respect to method 100.

In some embodiments, the lattice 12 or the hierarchical lattice 20 maybe patterned by applying (e.g., printing using a printer 212) a solutionthat selectively dissolves or removes at least a portion of theparticles 20. The lattice 12 or the hierarchical lattice 20 may bepatterned with the solution to form grooves, ridges, or channels 22 inthe lattice 12 or the hierarchical lattice 20. The grooves, ridges, orchannels 22 may be spaced in a fixed pattern across the surface, orvaried irregularly to form a desired printed shape and/or pattern. Thesolution may comprise an organic solvent that dissolves the particles10, such as toluene or ethyl acetate.

The method 200 further includes a step 206 of applying the biocompatiblepolymer solution to the hierarchical lattice 20 such that thebiocompatible polymer solution fills voids between the plurality ofparticles 10. The biocompatible polymer solution is then dried for aduration (e.g., 24 h at 25° C. and 30% relative humidity) to form a film24 (e.g., biocompatible polymer/particle composite) that encloses theparticles 10. As indicated by step 210, the particles 10 are thenremoved (e.g., by selectively printing an organic solvent onto the film24 using the printer 212), thereby leaving periodic cavities or voids 28in the film 24 to form a hierarchical opal 26 having a structural color.The hierarchical opal 26 produced using method 200 may have the sameproperties as described with respect to method 100.

In some embodiments, the method 200 further includes a step ofpatterning the surface of the hierarchical opal 26. Patterning thesurface of the hierarchical opal 26 may be achieved by printing asolution onto the surface of the hierarchical opal 26. Suitablesolutions include solvents that dissolve the biocompatible polymer, orsolvents that alter the lattice constant of the hierarchical opal 26 inthe printed region, thereby forming grooves, ridges, and/or channels 22on the surface of the hierarchical opal 26. Suitable solutions include,but are not limited to, aqueous solutions comprising an alcohol, such asmethanol, ethanol, propanol, and butanol. In some embodiments, thealcohol is present at a concentration from about 1% (w/w) to about 10%(w/w).

In some embodiments, the grooves, ridges, and/or channels 22 generatedusing method 200 have a width that ranges from about 10 μm to about 1mm. In some embodiments, the groove width or spacing may be at leastabout 10 μm, at least about 20 μm, at least about 30 μm, at least about40 μm, at least about 50 μm, at least about 60 μm, at least about 70 μm,at least about 80 μm, at least about 90 μm, at least about 100 μm, atleast about 125 μm, at least about 150 μm, at least about 175, at leastabout 200 μm, at least about 225 μm, at least about 250 μm, at leastabout 275, at least about 300 μm, at least about 325 μm, at least about350 μm, at least about 375, at least about 400 μm, at least about 425μm, at least about 450 μm, at least about 475, at least about 500 μm, atleast about 525 μm, at least about 550 μm, at least about 575 μm, atleast about 600 μm, at least about 650 μm, at least about 700 μm, atleast about 750 μm, at least about 800 μm, at least about 850 μm, atleast about 900 μm, at least about 950 μm, at least about 1000 μm, ormore.

In some embodiments, the grooves, ridges, and/or channels 22 have awidth that is at most 1.5 mm, or at most 1.4 mm, or at most 1.3 mm, orat most 1.2 mm, or at most 1.1 mm, or at most 1 mm, or at most 900 μm,or at most 800 μm, or at most 700 μm, or at most 600 μm, or less.

In some embodiments, the method 200 includes post processing thehierarchical opal 26 by exposing the hierarchical opal to water vapor orUV radiation, as described in method 100.

Apparatus

In some embodiments, the present disclosure provides a hierarchical opal26, and specifically a silk fibroin hierarchical opal 26. Thehierarchical opal 26 may be produced using the foregoing methods 100 and200. In some embodiments, the hierarchical opal 26 exhibits structuralcolor when exposed to incident electromagnetic radiation. Thehierarchical opal 26 comprises periodic cavities 28 or voids separatedby a lattice constant. The hierarchical opal 26 includes a surfacehaving grooves, ridges, and/or channels 22.

In some embodiments, the periodic cavities 28 may have an averagediameter that ranges from about 5 nm to about 2000 nm, or more. Forexample, the particles 10 and periodic cavities 28 may have an averagediameter of at least about 5 nm, at least about 10 nm, at least about 15nm, at least about 20 nm, at least about 25 nm, at least about 30 nm, atleast about 35 nm, at least about 40 nm, at least about 50 nm, at leastabout 60 nm, at least about 70 nm, at least about 80 nm, at least about90 nm, at least about 100 nm, at least about 125 nm, at least about 150nm, at least about 175, at least about 200 nm, at least about 225 nm, atleast about 250 nm, at least about 275, at least about 300 nm, at leastabout 325 nm, at least about 350 nm, at least about 375, at least about400 nm, at least about 425 nm, at least about 450 nm, at least about475, at least about 500 nm, at least about 525 nm, at least about 550nm, at least about 575, at least about 600, at least about 650, at leastabout 700, at least about 750, at least about 800, at least about 850,at least about 900, at least about 950, at least about 1000, at leastabout 1500, or at least about 2000 nm or more. In some embodiments theperiodic cavities have a spherical shape.

In some embodiments, the periodic cavities 28 may have an averagediameter that is at most 2000 nm, or at most 1500 nm, or at most 1400nm, or at most 1300 nm, or at most 1200 nm, or at most 1100 nm, or atmost 1000 nm, or at most 900 nm, or at most 800 nm, or at most 700 nm,or at most 600 nm, or at most 500 nm, or less.

In some embodiments, the lattice constant of the hierarchical opal 26may range from about 5 nm to about 2000 nm, or more. For example,lattice constant may be at least about 5 nm, or at least about 10 nm, orat least about 15 nm, at least about 20 nm, at least about 25 nm, atleast about 30 nm, at least about 35 nm, at least about 40 nm, at leastabout 50 nm, at least about 60 nm, at least about 70 nm, at least about80 nm, at least about 90 nm, at least about 100 nm, at least about 125nm, at least about 150 nm, at least about 175, at least about 200 nm, atleast about 225 nm, at least about 250 nm, at least about 275, at leastabout 300 nm, at least about 325 nm, at least about 350 nm, at leastabout 375, at least about 400 nm, at least about 425 nm, at least about450 nm, at least about 475, at least about 500 nm, at least about 525nm, at least about 550 nm, at least about 575, at least about 600, atleast about 650, at least about 700, at least about 750, at least about800, at least about 850, at least about 900, at least about 950, atleast about 1000, at least about 1500, or at least about 2000 nm ormore.

In some embodiments, the lattice constant of the hierarchical opal 26may have an average diameter that is at most 2000 nm, or at most 1500nm, or at most 1400 nm, or at most 1300 nm, or at most 1200 nm, or atmost 1100 nm, or at most 1000 nm, or at most 900 nm, or at most 800 nm,or at most 700 nm, or at most 600 nm, or at most 500 nm, or less.

In some embodiments, the grooves, ridges, and/or channels 22 on thesurface of the hierarchical opal 26 define a diffraction gratingseparated by a groove spacing. In some embodiments, the groove spacingin the hierarchical structure ranges from about 600 lines/mm to about3600 lines/mm. In some embodiments, the groove spacing is at least about600 lines/mm, or at least about 700 lines/mm, or at least about 800lines/mm, or at least about 900 lines/mm, or at least about 1000lines/mm, or at least about 1100 lines/mm, or at least about 1200lines/mm, or at least about 1300 lines/mm, or at least about 1400lines/mm, or at least about 1500 lines/mm, or at least about 1600lines/mm, or at least about 1700 lines/mm, or at least about 1800lines/mm, or at least about 1900 lines/mm, or at least about 2000lines/mm, or at least about 2100 lines/mm, or at least about 2200lines/mm, or at least about 2300 lines/mm, or at least about 2400lines/mm, or at least about 2500 lines/mm, or at least about 2600lines/mm, or at least about 2700 lines/mm, or at least about 2800lines/mm, or at least about 2900 lines/mm, or at least about 3000lines/mm, or at least about 3100 lines/mm, or at least about 3200lines/mm, or at least about 3300 lines/mm, or at least about 3400lines/mm, or at least about 3500 lines/mm, or at least about 3600lines/mm.

In some embodiments, the groove spacing in the hierarchical structure 26is at most 3600 lines/mm, or at most 3200 lines/mm, or at most 2800lines/mm, or at most 2400 lines/mm, or at most 2000 lines/mm, or at most1600 lines/mm, or at most 1200 lines/mm, or less.

In some embodiments, the grooves, ridges, and/or channels 22 of thehierarchical opal 26 have a groove spacing or width that ranges from 50nm to 30 μm. In some embodiments, the groove spacing or width may be atleast about 50 nm, at least about 60 nm, at least about 70 nm, at leastabout 80 nm, at least about 90 nm, at least about 100 nm, at least about125 nm, at least about 150 nm, at least about 175, at least about 200nm, at least about 225 nm, at least about 250 nm, at least about 275, atleast about 300 nm, at least about 325 nm, at least about 350 nm, atleast about 375, at least about 400 nm, at least about 425 nm, at leastabout 450 nm, at least about 475, at least about 500 nm, at least about525 nm, at least about 550 nm, at least about 575 nm, at least about 600nm, at least about 650 nm, at least about 700 nm, at least about 750 nm,at least about 800 nm, at least about 850 nm, at least about 900 nm, atleast about 950 nm, at least about 1000 nm, or at least about 2 μm, orat least about 3 μm, or at least about 4 μm, or at least about 5 μm, orat least about 10 μm, or at least about 15 μm, or at least about 20 μm,or at least about 25 μm, or at least about 30 μm.

In some embodiments, the grooves, ridges, and/or channels 22 of thehierarchical opal 26 have a groove spacing or width that is at most 30μm, or at most 25 μm, or at most 20 μm, or at most 15 μm, or at most 10μm, or at most 5 μm, or at most 1000 nm, or at most 800 nm, or at most600 nm, or at most 400 nm, or less.

In some embodiments, the grooves, ridges, and/or channels 22 have awidth that ranges from about 10 μm to about 1 mm. In some embodiments,the groove width or spacing may be at least about 10 μm, at least about20 μm, at least about 30 μm, at least about 40 μm, at least about 50 μm,at least about 60 μm, at least about 70 μm, at least about 80 μm, atleast about 90 μm, at least about 100 μm, at least about 125 μm, atleast about 150 μm, at least about 175, at least about 200 μm, at leastabout 225 μm, at least about 250 μm, at least about 275, at least about300 μm, at least about 325 μm, at least about 350 μm, at least about375, at least about 400 μm, at least about 425 μm, at least about 450μm, at least about 475, at least about 500 μm, at least about 525 μm, atleast about 550 μm, at least about 575 μm, at least about 600 μm, atleast about 650 μm, at least about 700 μm, at least about 750 μm, atleast about 800 μm, at least about 850 μm, at least about 900 μm, atleast about 950 μm, at least about 1000 μm, or more.

In some embodiments, the grooves, ridges, and/or channels 22 have awidth that is at most 1.5 mm, or at most 1.4 mm, or at most 1.3 mm, orat most 1.2 mm, or at most 1.1 mm, or at most 1 mm, or at most 900 μm,or at most 800 μm, or at most 700 μm, or at most 600 μm, or less.

In some embodiments, the groove spacing or width may be fixed.Alternatively, the groove spacing or width may be irregular (i.e.,varies throughout along a dimension of the hierarchical opal 26).

In some embodiments, the grooves, ridges, and/or channels 22 of thehierarchical opal 26 have a groove depth or height that ranges from 50nm to 1 μm. The groove depth or height may be defined as a distance orlength from the top of the groove, ridge, or channel relative to asurface of the hierarchical opal 26. In some embodiments, the groovedepth may be at least about 50 nm, at least about 60 nm, at least about70 nm, at least about 80 nm, at least about 90 nm, at least about 100nm, at least about 125 nm, at least about 150 nm, at least about 175, atleast about 200 nm, at least about 225 nm, at least about 250 nm, atleast about 275, at least about 300 nm, at least about 325 nm, at leastabout 350 nm, at least about 375, at least about 400 nm, at least about425 nm, at least about 450 nm, at least about 475, at least about 500nm, at least about 525 nm, at least about 550 nm, at least about 575 nm,at least about 600 nm, at least about 650 nm, at least about 700 nm, atleast about 750 nm, at least about 800 nm, at least about 850 nm, atleast about 900 nm, at least about 950 nm, or at least about 1000 nm.

In some embodiments, the grooves, ridges, and/or channels 22 of thehierarchical opal 26 have a groove depth or height that is at most 3 μm,or at most 2 μm, or at most 1 μm, or at most 800 nm, or at most 600 nm,or at most 400 nm, or less.

In some embodiments, the hierarchical opal 26 comprises from two tofive-hundred layers of periodic cavities 28, or more. In someembodiments the hierarchical opal comprise at least 2 layers, at least 3layers, at least 4 layers, at least 5 layers, at least 6 layers, atleast 7 layers, at least 8 layers, at least 9 layers, at least 10layers, at least 20 layers, at least 30 layers, at least 40 layers, atleast 50 layers, at least 60 layers, at least 70 layers, at least 80layers, at least 90 layers, at least 100 layers, or more.

In some embodiments, the hierarchical opal 26 comprises at most 1000layers, or at most 800 layers, or at most 600 layers, or at most 400layers, or at most 200 layers, or at most 100 layers, or at most 10layers of periodic cavities 28.

In some embodiments, the hierarchical opal 26 is substantially free oforganic solvent (e.g., toluene and/or ethyl acetate).

In some embodiments, the hierarchical opal 26 includes an additive at aconcentration from 0.1% (w/w) to 30% (w/w). In some embodiments,hierarchical opal 26 includes plasmonic particles or nanoparticlesinclude, but are not limited to, gold, silver, ruthenium, rhodium,palladium, osmium, iridium, platinum, titanium, aluminum, nickel,fluorine, cerium, tin, bismuth, antimony, molybdenum, chromium, cobalt,zinc, tungsten, polonium, rhenium and copper.

In some embodiments, the hierarchical opal 26 is or comprises a silkfibroin hierarchical opal 26. At least a portion of the hierarchicalopal 26 may be characterized by a percent beta sheet structure withinthe range of about 0% to about 45%. In some embodiments, silkfibroin-based stents are characterized by crystalline structure, forexample, comprising beta sheet structure of at least about 1%, at leastabout 2%, at least about 3%, at least about 4%, at least about 5%, atleast about 6%, at least about 7%, at least about 8%, at least about 9%,at least about 10%, at least about 11%, at least about 12%, at leastabout 13%, at least about 14%, at least about 15%, at least about 16%,at least about 17%, at least about 18%, at least about 19%, at leastabout 20%, at least about 21%, at least about 22%, at least about 23%,at least about 24%, at least about 25%, at least about 26%, at leastabout 27%, at least about 28%, at least about 29%, at least about 30%,at least about 31%, at least about 32%, at least about 33%, at leastabout 34%, at least about 35%, at least about 36%, at least about 37%,at least about 38%, at least about 39%, at least about 40%, at leastabout 41%, at least about 42%, at least about 43%, at least about 44%,or at least about 45%.

In some embodiments, the hierarchical opal 26 includes a portion of thesilk having a beta structure of at most 65% (w/w), or at most 60%, or atmost 55%, or at most 50%, or at most 45%, or at most 40%, or at most35%, or at most 30% (w/w).

In some embodiments, the present disclosure provides a firsthierarchical opal having a least one layer that forms grooves, ridges,and or channels 22 that is coupled to a second inverse opal through anadhesive layer. The second inverse opal may include a surface that isflat or substantially flat. In some embodiments, the second inverse opalis a silk fibroin opal. In some embodiments, the adhesive layer includesoptically clear adhesive. In some embodiments, the adhesive layerincludes silk fibroin (e.g., a silk fibroin film).

EXAMPLES

The following examples illustrate some embodiments and aspects of thedisclosure. It will be apparent to those skilled in the art that variousmodifications, additions, substitutions, and the like can be performedwithout altering the spirit or scope of the disclosure, and suchmodifications and variations are encompassed within the scope of thedisclosure as defined in the claims which follow. The following examplesdo not in any way limit the disclosure.

Example 1

Hierarchical Opal (HOP) Preparation:

In Example 1, a topographical templating strategy is provided for thefabrication of a biocompatible polymer-based, hierarchical 3D photonic.The provided biomaterials-based approach generates diffractive opticscomposed of nanophotonic lattices that allow simultaneous control overthe reflection (through the photonic bandgap) and the transmission(through 2D diffractive structuring) of light with the additionalutility of being constituted by a biocompatible, implantable, ediblecommodity textile material. The use of biocompatible polymers allowsadditional degrees of freedom in photonic bandgap design throughdirected protein conformation modulation. Demonstrator structures arepresented to illustrate the lattice multifunctionality, includingtunable diffractive properties, increased angle of view of photoniccrystals, color-mixing, and sensing applications.

In Example 1, a hierarchical opal is provided that may be used in avariety of applications including, but not limited to, includingsensing, displays, security, wetting, photovoltaics, aesthetics andothers.

The HOP was prepared by using a template of polystyrene (PS) colloidalcrystal on a topographically patterned substrate having a diffractiongrating. A crystalline PS nanosphere monolayer at the water/airinterface, generated through the direct self-assembly of PS nanospheres(diameters of 210 or 300 nm) is transferred onto pre-designeddiffractive surfaces to form templated, close-packed PS colloidalcrystal monolayers. The transfer process is repeated ultimatelyproviding a template lattice with a controllable number of layers.

Briefly, a monodisperse PS sphere suspension (4%) was introduced to thewater surface through using partially immersed hydrophilic Si wafer toform a floating monolayer. Then, large-scale close-packed monolayerarray was formed at the water/air interface after removing the spheresimmersed into the subphase of water and extra adding a few drops ofsodium dodecyl sulfate (SDS). The polycarbonate micro and nanopatternedsubstrate (Digital Optics, Tessera Inc., San Jose, Calif.) attached tosilicon wafer was immersed into the subphase to transfer the monolayerfrom the water surface. After repeating these procedures, hierarchicalcolloidal crystal multilayers were obtained.

The silk solution (30 min boil, 7 wt %-8 wt %) was prepared, and wasadded to the hierarchical colloidal crystal to fill all the air voids.The sample was set to dry for 24 h (25° C., 30% relative humidity) toform a free-standing patterned silk/PS composite film. The HOP film withthickness of ˜50 μm was finally obtained after removing all the PSspheres in toluene. The resulting structures have three scale-dependentoptical responses, namely (i) a photonic crystal behavior derived fromthe nanoscale periodicity of the optical lattice, (ii) the diffractivebehavior from the microscale patterning and (iii) lightgathering/processing ability derived from the multi-centimeter size ofthe devices.

Hierarchical opals were generated by using three kinds of templatingdiffractive optical elements: a diffuser (D), a pattern generator (PG),and a grating (G) (FIG. 5). The resulting HOPs are labeled as D-HOP,PG-HOP, and G-HOP to represent the 2D-diffusive/diffractive functionassociated with the photonic crystal lattice. Scanning electronmicroscope (SEM) images of the prepared silk HOPs are shown in FIG.6(A-C). At the microscale, HOPs can fully replicate the microstructuresof the corresponding substrate.

At the nanoscale, all hierarchical opals showed the ordered hexagonalarrays of air cavities (where the PS spheres were originally located)while displaying the ability to effectively replicate the surfacepatterns used as the template, as shown by the grating with d=600lines/mm and groove depth of 450 nm observed in the AFM images andcross-section profiles (FIG. 5).

The mismatch between the dimension of the nanoparticles and the gratingperiod can cause some edge irregularities in the end structures whichresults in the slight mismatch in depth and width of the colloidalassembly (FIG. 7). In this Example, the lattice constant (Λ), defined asthe centre-to-centre distance of the air cavities, is the same as thediameter of PS sphere used, i.e. Λ=300 nm (FIG. 6) and Λ=210 nm (FIG.7), respectively.

The cross-sectional SEM images of HOPs (FIG. 8) display the coexistenceof microscale and nanoscale features with the patterned diffractivestructure and the inverse photonic lattice both clearly visible. Thefidelity of these free-standing constructs is enabled by the materialcharacteristics of silk fibroin, which on top of its favorable opticalproperties (e.g., silk fibroin's robust mechanical properties andnanoscale processability). All the HOPs considered in this Example arethree-layered inverse colloidal crystals with a lattice constant of300/210 nm, unless otherwise noted.

The nanoscale periodic lattice structure of HOPs is responsible for thematerial's structural color as previously shown for silk inverse opals.The total reflectance spectra of both D-HOP and PG-HOP clearly showreflective stop-band peaks centered at λ=580 nm (FIG. 9A-B), and λ=420nm (FIG. 10) for Λ=300 nm and Λ=210 nm, respectively. Expectedly, thesehigh reflectance regions are more efficient with the increase of layernumbers of the inverse colloidal crystals (FIG. 11). The concomitantintroduction of microscale patterns can not only modulate the structuralcolor of HOPs, but maintain the performance of the diffractive opticalelements with the added utility of the interplay between reflective anddiffusive/diffractive functionality. The far-field diffraction patternsin both reflection and transmission obtained by propagating a laser beamthrough the HOPs are shown in FIG. 9(C-F) along with the AFM images ofthe inverse opals' diffractive surfaces. Additional functionality isillustrated in FIG. 9(G), which shows the images produced when an object(i.e. the word “Tufts” from an LCD display) is viewed through differentHOPs. As shown in FIG. 9(H-I), the image is either defocused ordiffracted by the D-HOPs and G-HOPs respectively, indicating thepreservation of the optical function caused by the templating andreplication of sub-micron topographies.

While the far-field intensity distribution of a diffractive opticalelement depends on its surface structure (e.g., height and periodicity),transparency, and effective refractive index, both the reflective andtransmissive contributions can be modulated by tuning the photonicbandgap associated with the inverse colloidal crystal lattice. As such,the number of layers in the colloidal photonic crystal may determine theoptical interplay in the HOP structures. The effect of the photoniccrystal lattice on the diffractive properties of PG-HOPs and D-HOPs wasevaluated FIG. 10(A) and FIG. 12(A-C) by comparing the far-fieldintensity of the diffracted orders of a HOP and an identical diffractivestructure without the photonic crystal lattice.

The laser wavelength was selected to match the stop-band peak positionto enhance the contribution due to the photonic crystal lattice. Asshown in FIG. 10(B), the reflected diffraction intensity increases,while the transmitted diffraction intensity decreases in correspondenceof the increase in the number of lattice layers due to the enhancedreflectivity from photonic crystal leading to higher diffractionefficiencies This is also illustrated in FIG. 10C, where the reflectedfirst-order diffraction efficiency from G-HOPs increases when thestop-band matches the laser wavelength, while the transmitteddiffraction efficiency decreases.

Tuning the stop-band of the photonic crystal introduces an additionaldegree of spectral selectivity on the diffractive structure thattemplates the lattice, which in turn can provide tunability andselectivity over the diffracted pattern from the structure. Theinfluence of the lattice constant on the diffractive properties of HOPswas first examined by assembling hierarchical opals with differentlattice constants Λ. Tuning the photonic lattice influences theextraction efficiency of specific wavelengths and allows one to enhancethe intensity of diffracted spectral components in reflection when thelaser wavelength matches the stop-band peak position.

This is confirmed by the measurements shown in FIG. 10D, whereillumination by a blue laser (λ=405 nm) causes the structure with Λ=210nm to display higher reflected diffraction intensity than thecorresponding structure with Λ=300 nm, while reflected intensities arelower for the Λ=210 nm lattice compared to the structure with Λ=300 nmspacing when the structure is illuminated by a green laser (λ=543.5 nm).Expectedly, the transmitted diffraction intensities correlate with whatobserved above. Similarly, G-HOPs with Λ=210 nm show lower +1^(st) orderreflected diffraction efficiency and then higher transmitted diffractionefficiency than the corresponding structure with Λ=300 nm whenilluminated by a green laser (FIG. 10C) due to the mismatched wavelengthbetween laser and stop-band peak.

Example 2

Hierarchical Opal Post-Treatment

The reconfigurability of the photonic lattice by UV or water vaporallows further degrees of freedom in the design of 2D/3D opticalstructures allowing for multispectral optimization of thediffracted/transmitted/reflected spectral components and theirinterplay. FIG. 10(E-F) shows an example of hierarchical structuresmodulation caused by UV or water vapor treatment on the diffractionperformance of the HOPs. One of the advantages of using biocompatiblepolymers, and specifically silk fibroin, is the ability to inducecontrollable conformational changes in the amorphous matrix of thematerial through the rearrangement of the fibroin molecular chains. Thisis achieved by using either UV or water vapor to modify the photoniclattice of silk inverse opal and allowing for programmable structuralcolor tuning. This unique feature applies to the hierarchical opalspresented here, where the same strategy can be adopted to providephotonic lattice tuning and further rational design of the material'sspectral response.

In Example 2, VL-215.G UV germicidal lamps with the wavelength of 254 nmand intensity of 76 μW cm⁻² were used for UV irradiation. The distancebetween sample and UV lamp was set as 1cm. Water vapor treatment wasperformed by directly exposing nanostructured surface of HOP to heatedwater surface (about 40° C.) with the distance between sample and watersurface of 5 mm. For post-patterned HOP, stencils (i.e., masks) withdesigned shapes were applied on the surface of HOP film before UV orwater vapor treatment to leave desired pattern on it after mask removal.

Conformational changes in the lattice result in controllable variationsof the diffraction intensity, consistent with the associated change inreflection intensity because of photonic crystal lattice modulation FIG.10(E-F). It is observed that while the lattice constant can be tuned,the surface micropatterns are almost unaffected after either UV or WVtreatment, as confirmed by the surface SEM images.

The diffraction properties of the G-HOP structures when illuminated by awhite light source are also shown in FIG. 10H, where the dispersedspectrum can be seen in several positive and negative diffractiveorders. Compared to the plain grating, the transmitted diffractionpatterns are affected by the photonic crystal transfer function whichfilters light in the stop-band. This is verified by analyzing thetransmitted spectrum in the m=1 diffracted order, which shows aconsistently lower transmitted intensity where the stop-band position ofthe inverse opal lattice is present with varying reflected intensity asa function of the number of layers in the photonic crystal lattice FIG.10(I).

The interplay between the photonic bandgap and 2D diffusion/diffractionaffects the overall iridescence of the structures, providing a strategyto enhance spectral selectivity in biocompatible polymer-based materialswhose index contrast is low compared to inorganics and generally do notpossess a complete photonic bandgap. This interplay has an impact on thereflected structural color at different angles. As an example, in thecase of D-HOP/PG-HOP, the vivid yellow color is still visible as theviewing angle is changed from normal to oblique FIG. 11(A) in contrastwith the unpatterned silk inverse opal (seen at the edge of eachelement), whose color is notably blue-shifted. The diffused reflectancespectra measurement (FIG. 11B) shows largely unchanged peak wavelengthsas the observation angle is increases. This underscores the diffusiveeffect of the micropatterns that results in increased viewing angles forthe nanoscale lattice's structural color.

Conversely, the color varies from blue to red with the increase ofobservation angle for a silk diffraction grating. This simplediffractive effect is altered by adding a photonic crystal lattice andby increasing the layer numbers of the photonic crystal. The observedstructural color of a five-layer G-HOP is nearly unchanged beyond acertain angle, in contrast to the angle-dependent iridescence of a plainsilk inverse opal. This provides another example of the coordinatedeffect between the angle-dependence of the 2D grating and the photoniccrystal iridescence, with the structural color from the stop-band of thephotonic crystal gradually dominating the response as the lattice getslarger with the increase of the number of assembled colloidal layers.

The integrated capacities to exhibit uniform structural color over abroad viewing angle and to locally reconfigure the structural colors andthen design multicolor patterns make D-HOPs or PG-HOPs be potentials forwide-angle pattern displays. To demonstrate this, we first generated amulticolor “silk” word and tree pattern by selectively exposing part ofHOP to UV and WV for different times and modulating the photonic crystallattice constant to display patterned structural color, as shown in FIG.11(D-E). A comparison between the PG-HOP with a tree pattern (FIG. 11F,top) shows little difference in structural color as the viewing angleincreased from the normal, in contrast to an unpatterned silk inverseopal where the angle-dependence of structural colors is evident (FIG.11F, bottom).

Example 3

Exemplary Apparatuses including Hierarchical Opals

The integrated capacities to exhibit uniform structural color over abroad viewing angle and to locally reconfigure the structural colors andthen design multicolor patterns make D-HOPs or PG-HOPs be potentials forwide-angle pattern displays. To demonstrate this, we first generated amulticolor “silk” word and tree pattern by selectively exposing part ofHOP to UV and WV for different times and modulating the photonic crystallattice constant to display patterned structural color, as shown in FIG.11(D-E). A comparison between the PG-HOP with a tree pattern FIG. 11F,top, shows little difference in structural color as the viewing angleincreased from the normal, in contrast to an unpatterned silk inverseopal where the angle-dependence of structural colors is evident FIG.11F, bottom.

Given the coexistence of spectrally responsive functions within a uniqueoptical element, these structures offer interesting opportunities forsensing by combining the features of photonic crystals and diffractiveoptical elements. Typically, photonic-crystal based sensors work onmonitoring stop-band spectral shifts, while diffractive opticalelements-based sensors commonly rely on the analysis of far-fielddiffraction pattern changes in response to outside stimuli. In Example3, HOPs allow for simultaneous monitoring of stop-band spectral shiftsand diffracted orders, combining the utility of both approaches.

As an example, a D-HOP is used to monitor the refractive index (RI)changes of IPA-glycerol mixtures. FIG. 12(A) shows the reflectancechange when the HOPs were immersed in IPA-glycerol mixtures with varyingcompositions. The performance of the sensor is shown in FIG. 12(B),which plots the wavelength and reflection intensity with differentvolume fraction of glycerol (Φ). An increase of the concentrations ofglycerol enhances the RI of the mixed solution and results in ared-shift of the stop-band, along with a decrease of reflectionintensity, for both HOPs with different lattice constants. Thediffraction patterns of D-HOPs as well as silk diffuser in differentIPA-glycerol mixtures and the corresponding diffraction performance as afunction of glycerol concentration are shown in FIG. 12(C-D). Thediffraction intensity of D-HOP decreases with increasing glycerolconcentration, the same as that of silk diffuser. This dual sensingbased on stop-band (wavelength shift and relative intensity change) anddiffraction (intensity change) can add utility for sensing applications,pending further research to improve selectivity and sensitivity.

These HOPs may be incorporated into implantable devices for biomedicalapplications, including biodegradation process and drug deliverymonitoring. Moreover, the easy implementation of high reflectivity inspecific wavelength (through increasing the layer numbers of inversecolloidal crystal) and high light diffusion makes the HOPs potentiallyas back reflectors of optoelectronic devices, such as photovoltaics,LED, and phototransistors, to enhance their performance throughincreasing light trapping and absorption within the devices. Finally,the protein feature of silk fibroin also enables these hierarchicalstructures to be readily transferred to other materials (such as Au,Al₂O₃, TiO₂ and so on) by removing the silk template at hightemperature, further opening new path for photonic crystal-basedoptoelectronics applications.

Example 4

Printed Hierarchical Opal Preparation

In Example 4, an inkjet-printing based strategy for the generation ofnon-contact, rapid, direct approaches to create arbitrarily patternedphotonic crystals is provided. The strategy is based on the use ofwater-soluble biocompatible polymer-based opal structures that can bereformed with high resolution through precise deposition of fluids onthe photonic crystal lattice. The resulting digitally designed photoniclattice formats simultaneously exploit structural color and materialtransient opening avenues for applications including, but not limitedto, information encoding, and combined functions of optics,biomaterials, and environmental interfaces in a single device. Thepatterned photonic crystals may also be utilized in sensors, colordisplays, optical devices, and anti-counterfeiting.

Two strategies are provided in Example 4 to generate patterned silkinverse opal (SIO) by using inkjet printing were developed, namely: (i)by printing ethyl acetate on polystyrene (PS) colloidal crystalmultilayers to generate patterned template for silk infiltration andsubsequent patterned inverse lattices, and (ii) by printing MeOH/waterdirectly on inverse opal lattice for the generation of multispectralpatterns.

To build the multilayer PhC template, three layers of PS colloidalcrystals (300 nm in diameter) were assembled on a silicon wafersubstrate using layer-by-layer transfer techniques, through whichlarger-scale, defect-free 3D colloidal photonic structures can beobtained. PS spheres self-assemble and form a crystalline monolayerfloating on the surface after being introduced into water.

In the first approach, ethyl acetate is inkjet printed (DimatixDMP-2831) on the 3-layer direct opal assembly thus dissolving thepolystyrene assembly along the printed area. The patterned directphotonic lattice can then be used as template to fabricate an inverseopal structure by infiltrating pure silk solution, which after dryingcan be immersed in toluene to remove the PS spheres and the PS solutefrom printing.

In the second approach, a silk inverse opal lattice is used as thesubstrate while the inkjet printer cartridge is loaded with a mixture ofmethanol (MeOH) and water. This approach leverages the previouslydescribed ability of directing conformational changes in the silkprotein lattice through humidity. Printing small volumes of water leadsto localized variations in the lattice constant of the SIO, while andultimately allows to “print” structural color by defining areas ofdifferent lattice constants through the water proportion in dropletsdeposited.

The use of the two approaches described above, allows for differentmechanisms of structural color patterning. In first approach, directdissolution of the direct opal lattice results in an inverseclose-packed face-centered-cubic (fcc) lattice except for the printedareas where the ethyl acetate completely eliminates the lattice,resulting in regions without structural color.

This drawing strategy can be utilized to create both direct and inversepatterns as shown in FIG. 13(A). Inkjet printing provides the ability totune the drop volume and spacing, the distance between printed lines,resulting in photonic features that can be controlled down to 30 μm insize as seen in FIG. 13(C). As a demonstration of the complexity of thepatterns achievable, a digitized image file was printed onto the directopal and then reproduced onto a silk film as an inverse opal FIG. 13(D).Analysis of the edges of the printed structures by scanning electronmicroscopy (SEM) shows the interface between the unaffected photoniclattice, exhibiting the ordered fcc-arrangement template by the PSnanospheres next to the patterned regions in which the lattice featuresare hardly and a smooth silk film surface is visible FIG. 13(D). A stepof ˜400 nm in height between the porous lattice structure and smoothsurface is observed in the atomic force microscopy (AFM) image.

This type of patterning, combined with the ability of the biomaterialsubstrate to be reconfigured and interfaced with the environment thatsurrounds it, can be used to add functionality to photonic crystalslattices not only from a spectral perspective but also as an informationencoding approach where the possibility to digitally definemultispectral lattices onto a biologically compatible substrate couldlend itself to added utility in encryption, storage, and security.

A simple demonstrator device in this context is a quick response (QR)code: first proposed in year 1994, and are ubiquitous with typicalapplications ranging from item identification, authentication,product/time tracking, and document management, to name a few. Comparedwith conventional barcodes, QR codes have more advantages including fastreadability, high data storage capacity in small printed space, andresistance to dirt and damage. The ever-increasing use of personalizedinterfaces, such as smartphone cameras, to read/access digitizedinformation, is driving the demand for even more sophisticated datastorage, encryption, and security. For this purpose, advancedmaterials/structures, such as plasmonic filters, colloidal photoniccrystals, upconverting nanoparticles, and metamaterials, have beenutilized to expand on this platform.

Compared with conventional barcodes, QR codes have more advantagesincluding fast readability, high data storage capacity in small printedspace, and resistance to dirt and damage. The ever-increasing use ofpersonalized interfaces, such as smartphone cameras, to read/accessdigitized information, is driving the demand for even more sophisticateddata storage, encryption, and security. For this purpose, advancedmaterials/structures, such as plasmonic filters, colloidal photoniccrystals, upconverting nanoparticles, and metamaterials, have beenutilized to expand on this platform. Combining these three functionsonto a single substrate it contemplated herein, and may provide utilityfor environmentally-dependent encryption and readout modalities.

The demonstrator device is generated by digitally printing a QR codeonto a direct photonic lattice and then replicated by infiltration ofsilk fibroin solution to generate a free-standing, water-soluble SIO QRcode. The resulting structure, shown in FIG. 14(A), preserves all therequired details to be successfully scanned and read out with asmartphone. The inverse opal structure generated on the silk substrateoffers an instantaneous way to exploit material transience and makesmessages disappear as a function of protein conformation and assembly.Silk films can be controlled to have programmable solubility as afunction of the physical crosslinks (i.e. prevalence of inter- vs.intramolecular bonds) presented in the material matrix.

In the example presented here, the silk substrate is assembled in itsamorphous state and thus in a water soluble form. As a result, thespectral photonic lattice can be wiped out by a simple wet finger touchdue to the sensitive response of the three-layer SIO structure (i.e. <1μm) to localized humidity. The 50 μm-thick silk film, as the messagecarrier, can also be entirely dissolved within 10 seconds (with the QRcode degrading in ˜100 ms) by immersing the film in water (FIG. 14A).The rapid wipeout suggests the ability to have an environmentallyinterfaced tag with a specific photonic lattice spectral response thatcan react to localized humidity or can be dissolved on command byexposure to water.

It should be noted that though the water-soluble Silk I structure inamorphous silk can transform to the crystalline β-sheet structure (SilkII) through various types of treatments, the silk films demonstratedhere can retain their amorphous structure and act as a reconfigurablemessage carrier with outstanding mechanical properties and flexibilityfor extended time.

In addition to simply fabricating a dissolvable QR code, the use ofbiocompatible polymer substrates allows to add additional layers offunctionality to these constructs. It was previously demonstrated thatin an amorphous silk-based inverse opal lattice, controlled exposure towater vapor (WV) can induce the rearrangement of free molecular chainsin silk, thus leading to controllable vertical compression of photoniclattice itself, ultimately reconfiguring the spectral response of thephotonic crystal. This strategy was adopted by generating amultispectral QR code by programming the photonic crystal lattice sothat the three open squares in the QR encoding show distinct blueshifted color after WV treatment FIG. 14(B-D). Measurements of thecorresponding reflectance spectra show that reflected peaks varying from550, 485, to 445 nm compared with the untreated pattern (600 nm) whenthe treating time varies from 1, 2, to 3s (FIG. 14C).

To decipher the messages encoded in FIG. 14D, two passwords are requiredand programmed in advance (i) the digital pattern scanned from the QRcode (1^(st) encryption layer) and (ii) a 3-digit structural color keyencoded in the photonic crystal and decoded from the three open squares(2^(nd) encryption layer). The raw image was processed into a binaryimage (i.e. black and white) by setting an intensity threshold, and thenthe colors were inverted to enable the camera scanning message retrieval(1^(st) encryption layer). The differences in reflective spectra amongthe three open squares (FIG. 14C) can be read out in terms of RGBintensity, from which the 2^(nd) encryption layer is extracted. As shownin FIG. 3D, each square can be defined as a digit based on custom lookuptable associated to the RGB intensity (e.g. the code extracted is 796 inthis case). The 3-digit code can thus provide the key to accompany theQR code and decipher its message.

As previously demonstrated, the molecular rearrangement of silk fibroincan be controlled by inducing the protein's conformational transitionand causing an associated compression of the SIO photonic lattice in thevertical direction. This enables patterning of the SIO through localchanges in the lattice constant using the crosslinking solvent as apaint ink. To this end, MeOH/water mixtures with different water ratioswere chosen as the ink to design multicolor patterned SIO. Differentwater proportions in the ink formulations result in varied increase insilk protein chains mobility, thus allowing for varied degrees ofprotein molecular chain rearrangement, which ultimately results indistinct volumetric changes of the silk matrix.

As shown in FIG. 15(A), six blue elephants with distinct differences inbrightness were generated by inkjet printing MeOH/water mixtures withvaried water ratio on a 10-layer SIO substrate. Variations in reflectedcolor can be ascribed to the change in lattice constant as a result ofvertical opal structure compression induced by the ink. Cross-sectionalSEM images correlate the collapse of the nanoscale photonic lattice to ablueshift in the SIO's spectural response (FIG. 15B). As the inkjetdroplets' water content increases from 5% (c) to 18% (f), the aircavities experience increased deformation showing a more oblate shapealong the [111] direction, which leads to increased vertical compressionof photonic lattices compared to the control sample.

As a result, the wavelength of reflection peak gradually blueshifts from550 nm to 450 nm accompanied by a gradual decrease of reflectanceintensity (FIG. 15C). Interestingly, the blue-shifted wavelength followsa linear relationship with the water volume ratio in the MeOH/water ink(FIG. 15D). This dependence of the reflection peak position on the watercontent enables local control of the structural color and multicolorpattern design. As a demonstration, a butterfly pattern with blue bodyand veins and dark blue wing periphery was generated (FIG. 15E) by usinginks with different water contents.

With this strategy, patterns with arbitrary shape can be directly drawnon biocompatible polymer-based photonic structures at differentbrightness level, and more color choices can be expected by manipulatingthe canvas configuration and the ink formulation. Such patternsgenerated can be easily fixed by inducing physical crosslinking of thesilk matrix with methanol treatment at ambient condition.

This example demonstrates the feasibility of using inkjet printing toachieve large-scale patterning on silk inverse opal photonic lattices intwo contactless ways by (i) depositing ethyl acetate to locally generatedefects on PS multilayer photonic templates for subsequent inverse opal,and (ii) depositing MeOH/water directly on SIO to achieve multispectralpatterns.

In both approaches, direct and inverse pattern designs can be written ona water-soluble biocompatible polymer substrate avoiding the use ofaggressive chemical reactions (e.g. surface wettability modification) orexternal stimuli (e.g. mechanical force, magnetic field,photolithography). By specifically taking advantage of silk fibroin'sstructural polymorphism such biophotonic lattice can undergomultispectral responses through molecular chain rearrangement allowingfor interesting strategies for transient message encryption.

Other than being a digital deposition tool, inkjet printing techniquealso add utility by providing a reactive etching tool through controlledvolume deposition and spatial interactions between the ink and thesubstrate by tuning ink composition, drop volume, and pattern alignment.The integration of optical functions, biomaterial interfaces, anddigital writing techniques in a single device not only bringsbiophotonics into applications in color display and cryptography, butalso gives provides insight for the use of inkjet printing as asynthesis tool in biomaterial engineering.

Polystyrene (PS) photonic crystal (PhC) template preparation: Threelayers of PS sphere arrays on wafer were prepared. PS sphere suspensionwas introduced to the water surface and later scooping transferred witha 60-minute Tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane (FOTS)treated silicon wafer. By repeating the process for three times, athree-layer PS PhC structure was created and the colloids on thesubstrate remain close-packed after being transferred from watersurface.

Silk fibroin solution preparation: Silk fibroin solution was prepared bycutting Bombyx mori silk cocoons, and boiling for 30 minutes in 0.02 MNa₂CO₃ to remove sericin. 9.3 M LiBr solution was added to theovernight-dried silk fibroin and stored at 60° C. to dissolve fibersinto aqueous solution. Pure silk solution (˜6%) was collected afterdialysis (Fisherbrand, MWCO 3.5K) for 48 hours.

Silk Inverse Opal (SIO) preparation: Silk solution was added to thepatterned PS PhC template to fill the air voids. The sample was set todry for 24 h (25° C., 30% relative humidity) to form a free-standingsilk/PS composite film with the thickness of 50 μm. The PS sphereswithin the composite film were removed later by immersing the film intotoluene for 24 h.

SIO patterning: Ethyl acetate (Fluka Analytical) was inkjet printed by aDimatix Material Inkjet Printer (DMP-2831, equipped with cartridges with21 μm nozzle diameter, FUJIFILM, Santa Clara, Calif., USA) using acustom designed waveform. Drop spacing was set as 20 μm, cartridgeheight was 500 μm, firing voltage 25V. MeOH/water ink was prepared as3%, 3.5%, 5%, 9.5%, 12% and 18% in volume of water. All inkjet printingwork was performed at room temperature (20-23° C.) with a relativehumidity around 30%.

Water vapor treatment: The SIO was put on top of the heated watersurface (about 40° C.) with the nanostructured side directly exposed towater vapor over a controlled time (1, 2, and 3 seconds). The distancebetween sample and water surface was set as 5 mm. Stencils with opensquares were applied on the surface of SIO film to leave desired colorafter mask removal.

Lena pattern design: the pixelated image file “Lena” was first digitallycreated in MATLAB by reversing binary pixel values in the original“Lena” from 0 to 1 or vice versa in a random group of 5.

Test Methods

Diffraction Patterns Characterization

Diffracted patterns were obtained by propagating a blue (405 nm) orgreen (543.5 nm) laser through the silk 2D optical elements or HOPs. Thedistance between the sample and the projection plane was ˜10 cm forrecording transmitted diffraction patterns and ˜30 cm for reflecteddiffraction patterns. For diffraction patterns of white light, the lightfrom a fiber (core diameters of 400 μm) focused by a lens was directedtoward samples. MATLAB was utilized to analyze the diffractionintensity. Relative diffraction intensity was utilized to evaluate thediffraction performance of HOPs, which is characterized by dividing thespecific diffraction intensity of HOPs by the diffraction signal ofcontrol sample without inverse colloidal crystal.

Diffraction Efficiency Measurements

The grating or G-HOP is illuminated by HeNe laser with a wavelength of543.5 nm (Melles Griot, IDEX Health & Science, LLC). The first-orderdiffraction signal (I_(1st)) was focused by a 5 cm focal length lens andmeasured by an optical power meter (PM100D, Thorlabs GmbH, Germany). Tenpoints were randomly picked and measured. The diffraction efficiency(DE) was calculated as:

${DE} = \frac{I_{1\; {st}} - I_{background}}{I_{incident} - I_{background}}$

where the intensity of the incident beam (I_(incident)) was measured atposition right before the sample. The background noise signal(I_(background)) was recorded when the laser is off, with other setupsbeing intact.

Surface and Cross-Sectional Scanning Electron Microscope (SEM) Images

Surface and cross-sectional SEM images were acquired with a ZeissSupra55VP. To analyze the cross-sectional structure, the samples werecleaved via cryofracture. AFM Images were obtained with a Cypher AFM(Asylum Research) in tapping mode using an Arrow UHF silicon probe(BRUKER, MPP-21120-10). Optical microscopy (BH-2, OLYMPUS) equipped witha camera (MicroPublisher 3.3 RTV, Qlmaging) was used to observe thestructural color and surface morphology. UV-vis-NIR spectrophotometer(V-570, Jasco) combined with an integrating sphere (ISV-469, Jasco) wasused to obtain total reflectance and transmittance spectra, while thespecular reflectance and transmittance spectra, the diffused reflectancespectra and the diffraction spectra were collected using a fibre-opticspectrometer (USB-2000, Ocean Optics). The angle-resolved spectra weremeasured by fixing the incident light (normal to the sample surface) andadjusting the detection angle with respect to the surface normal.

It is specifically intended that the present disclosure not be limitedto the embodiments and illustrations contained herein, but includemodified forms of those embodiments including portions of theembodiments and combinations of elements of different embodiments ascome within the scope of the following claims.

1. A method of forming a hierarchical opal configured to separatewavelengths of light using a diffractive optical element, the methodcomprising the following steps: a) applying a silk fibroin solution to alattice comprising a plurality of particles, the lattice forming a moldfor the hierarchical opal having the diffractive optical element,wherein applying the silk fibroin solution fills voids between theplurality of particles; b) drying the silk fibroin solution into acomposite material including the hierarchical opal and the plurality ofparticles; and c) removing the plurality of particles to form thehierarchical opal having the diffractive optical element formed on asurface of the hierarchical opal.
 2. The method of claim 1, wherein themethod further comprises: a′) inducing a plurality of particles toassemble into the lattice on a surface of a substrate, the latticehaving a surface that defines the diffractive optical element configuredto separate wavelengths of light.
 3. The method of claim 1, wherein thediffractive optical element comprises at least one of a Fresnel lens, amicro lens, a pattern generator, a diffuser, a diffraction grating, abeamsplitter, or a beam displacement optic.
 4. The method of claim 2,wherein the surface of the substrate comprises a patterned surface. 5.The method of claim 4, wherein the diffractive optical element on thelattice is formed as a negative imprint of the patterned surface on thesubstrate.
 6. The method of claim 1, wherein a structure of thediffractive optical element is present in each layer of the lattice. 7.The method of claim 1, wherein step c) further comprises forming aplurality of layers having periodic cavities that form in the space ofthe removed particles.
 8. The method of claim 7, wherein a structure ofthe diffractive optical element is present in each of the plurality oflayers having the periodic cavities.
 9. The method of claim 2, whereinthe substrate comprises a silicon wafer.
 10. The method of claim 1,wherein the silk fibroin solution comprises silk fibroin at aconcentration of about 0.1% (w/w) to 30% (w/w).
 11. The method of claim1, wherein the particles comprise polystyrene or poly(methylmethacrylate) nanoparticles.
 12. The method of claim 1, wherein theparticles have an average diameter from about 100 nm to about 600 nm.13. The method of claim 2, wherein the step a′) further comprisesperforming layer-by-layer deposition to form a groove depth from about50 nm to 1 μm in the surface of the lattice.
 14. The method of claim 1,wherein the step c) further comprises forming a lattice constant in arange from 100 nm to 600 nm in the lattice.
 15. The method of claim 1,wherein the step c) further comprises immersing the lattice in anorganic solvent.
 16. The method of claim 1, wherein the silk solutionincludes dispersed plasmonic nanoparticles in the silk solution.
 17. Themethod of claim 12, wherein the plasmonic nanoparticles are selectedfrom the group consisting of gold, silver, ruthenium, rhodium,palladium, osmium, iridium, platinum, titanium, aluminum, nickel,fluorine, cerium, tin, bismuth, antimony, molybdenum, chromium, cobalt,zinc, tungsten, polonium, rhenium and copper.
 18. The method of claim 3,wherein the diffractive grating comprises an echellete grating, alittrow grating, or a holographic grating on the surface of the lattice.19. The method of claim 1 further comprising: exposing at least aportion the hierarchical opal to water vapor or ultraviolet radiationfor a duration.
 20. The method of claim 15, wherein the duration issufficient to alter the photonic band gap of the at least a portion ofthe hierarchical opal.
 21. The method of claim 15, wherein the durationof exposing the hierarchical opal to water vapor is from about 1 secondto about 10 seconds.
 22. The method of claim 15, wherein the duration ofexposing the hierarchical opal to ultraviolet radiation is from about 1second to about 5 hours.
 23. The method of claim 15, wherein theduration is sufficient to induce a beta-sheet content in the silkfibroin from 20% to 45% (w/w).
 24. The method of claim 1, wherein thediffractive optical element comprises grooves on a surface of thehierarchical opal having a width that ranges from 100 nm to 1 μm.25. 25.A method of forming a hierarchical opal, the method comprising: a)applying the silk fibroin solution to a lattice comprising a pluralityof particles such that the silk fibroin solution fills voids between theplurality of particles; b) drying the silk fibroin solution into acomposite material including the hierarchical opal and the plurality ofparticles; c) removing the plurality of particles to form thehierarchical opal comprising nanoscale periodic cavities separated by alattice constant; and d) applying an aqueous solution to a surface ofthe hierarchical opal in a patterned formation, wherein the solutionalters the lattice constant of the nanoscale periodic cavities locatedin the patterned formation to generate grooves across the surface of thehierarchical opal.
 26. The method of claim 25, wherein the aqueoussolution comprises an alcohol.
 27. The method of claim 26 wherein thealcohol is selected from methanol, ethanol, propanol, and butanol. 28.The method of claim 26, wherein the alcohol is present in the solutionat a concentration of 1% (w/w) to 10% (w/w).
 29. The method of claim 25,wherein d) includes printing the solution across the surface of thehierarchical opal.
 30. The method of claim 25, wherein the grooves havea width that ranges between 10 μm to 1 mm.
 31. The method of claim 26,wherein the width ranges between 10 μm and 500 μm.
 32. The method ofclaim 25, wherein the step c) further includes printing an organicsolution onto the silk film.
 33. The method of claim 32, wherein theorganic solution is selected from ethyl acetate and toluene.
 34. Themethod of claim 25, wherein the silk fibroin located within thepatterned formation comprises a beta-sheet content from 20% to 45%(w/w).
 35. An apparatus comprising: a hierarchical silk fibroin opalthat exhibits structural color when exposed to incident electromagneticradiation, the hierarchical silk fibroin opal comprising nanoscaleperiodic cavities separated by a lattice constant, wherein thehierarchical silk fibroin opal includes a surface having grooves. 36.The apparatus of claim 35, wherein at least a portion of the grooveshave a groove depth of 50 nm or greater.
 37. The apparatus of claim 36,wherein the groove depth ranges from 50 nm to 5 μm.
 38. The apparatus ofclaim 35, wherein the grooves form a diffractive optical element on thesurface of the hierarchical silk fibroin opal.
 39. The apparatus ofclaim 38, wherein the hierarchical silk fibroin opal comprises aplurality of layers, wherein each of the layers includes a structurecorresponding to the diffractive optical element.
 40. The apparatus ofclaim 39, wherein the diffractive optical element comprises at least oneof a Fresnel lens, a micro lens, a pattern generator, a diffuser, adiffraction grating, a beamsplitter, or a beam displacement optic. 41.The apparatus of claim 35, wherein the grooves have a groove spacingthat forms a repetitive grating structure.
 42. The apparatus of claim35, wherein the grooves have a width that ranges from 100 nm to 30 μm.43. The apparatus of claim 41, wherein the repetitive grating structureforms a ruled grating or a holographic grating across the surface of thehierarchical silk fibroin opal.
 44. The apparatus of claim 35, whereinthe grooves have a width that is greater than 30 μm.
 45. The apparatusof claim 44, wherein the grooves have a width that ranges between 30 μmand 1 mm.
 46. The apparatus of claim 35, wherein the nanoscale periodiccavities have a spherical shape.
 47. The apparatus of claim 35, whereinthe nanoscale periodic cavities have substantially a same diameter. 48.The apparatus of claim 35, wherein the hierarchical silk fibroin opalhas an average lattice constant in a range of between about 100 nm andabout 600 nm.
 49. The apparatus of claim 35, wherein at least onedimension of the apparatus is greater than a centimeter.
 50. Theapparatus of claim 35, wherein the apparatus comprises multiple layersof nanoscale periodic cavities.
 51. The apparatus of claim 35, whereinthe hierarchical opal comprises from two layers to one-hundred layers ofnanoscale periodic cavities.
 52. The apparatus of claim 35, wherein thehierarchical opal is substantially free of organic solvent.
 53. Theapparatus of claim 52, wherein the hierarchical opal is substantiallyfree of toluene and/or ethyl acetate.
 54. The apparatus of claim 35,wherein at least a portion of the hierarchical opal includes silkfibroin having a beta-sheet content that ranges from 20% to 45% (w/w).55. The apparatus of claim 35, wherein a lattice constant for at leastsome of the nanoscale periodic cavities of the hierarchical opal aresmaller in a vertical direction following exposure to water vapor orultra violet radiation.
 56. The apparatus of claim 35 furthercomprising: a first hierarchical silk fibroin opal having at least onelayer that forms the grooves in the first hierarchical silk fibroinopal; and a second inverse opal coupled to the first hierarchical silkfibroin opal through an adhesive layer.
 57. The apparatus of claim 56,wherein the adhesive layer comprises silk fibroin.
 58. The apparatus ofclaim 56, wherein the second inverse opal includes a surface that isflat.
 59. The apparatus of claim 58, wherein the second inverse opal isa silk fibroin opal.